This seems like an awkward implementation for users. After setting up sub categories and then later wanting to move them, I ran across this level restriction. I had to adjust my implementation as a result.
And now I see that it’s possible to have more levels … but (a) the feature is hidden and undocumented and (b) it’s still limited to 3.
I appreciate the update and work, but just don’t understand the restrictions and how they’re supposed to make sense to implementers.